It is well-known that lubricants tend to oxidize when exposed to adverse conditions. In the internal-combustion engine, the temperature of operation, for example, is an important variable since the oxidation rate increases with an increase in temperature. The formulation of lubricants to meet the most severe type of engine operation has been a great challenge, particularly in the area of multigraded oil for diesel engine applications. Additionally, with higher temperatures experienced in gasoline engines resulting from use of pollution equipment and a trend to small, high-revving engines, greater importance is attached now to more thermally and oxidatively stable lubricants for these applications. Thus, fluids which are conventionally conceived for mild diesel applications may find utility in the most severe gasoline engine operations.
Diesel engines, which are used to power a variety of heavy duty vehicles, such as trucks, buses, farm tractors, construction equipment and trains, generally impose severe high temperature service on their lubricants. The temperatures in the upper ring belt zone of the piston reach 500.degree.-700.degree. F. for diesel eqiupment in mild and high supercharged versions; whereas, temperature do not generally exceed 500.degree. F. in gasoline engines.
Correct operation of the diesel engine requires that the piston be properly lubricated and remain free to travel unimpeded in the cylinder, compression being maintained by means of the secure-fitting rings located in the piston grooves. These rings act as lubrication pumps and must be able to move vertically as well as circumferentially in the groove. When the fluid is exposed to the high temperatures of the top groove, decomposition occurs proportionate to the thermal/oxidative stability of the components of the fluid. This decomposition results in resinous, carbonaceous deposits in the groove(s) and on the lands of the piston. If sufficient buildup of deposits occurs, ring sticking results with eventual compression loss, piston scuffing or piston seizure. Thus, an important element in the quality control for a diesel fluid is the specification of a maximum amount of deposit in the top groove and other locations.
Historically, single-grade oils have occupied a preeminent position as recommended fluids for diesel engine application, particularly 30 and 40 weight oils whose base stocks are less volatile than lower SAE grades and would be less likely to volatilize in the hot zones of the piston. Multigraded diesel oils have not achieved acceptance, although diesel engine operation in cold weather would indicate a significant advantage with multigrades. Diesel engines are usually difficult to start, especially in cold weather. The cold-start problems are now remedied by the use of external heaters, an added expense and inconvenience, the use of ether which can result in engine damage since this extremely volatile solvent provides an explosive start with piston movement in the absence of lubrication, or by permitting continuous running when not in use which is wasteful of fuel and harmful from an environmental standpoint. Environmental and energy related considerations, therefore, could lead to a significant demand for multigraded diesel lubricants.
It is widely believed in the petroleum industry that the presence of polymer degrades diesel performance by increasing the amounts of Top Groove Fill (TGF) and lacquer. Their deficiency is the principal reason today that less than ten percent of the diesel lubricants used in this country are multigraded. For the major market, two performance levels are defined, Classification CC (Caterpillar 1-H) and Classification CRD (Caterpillar 1-G), with the 1-G level representing more severe, highly supercharged, engine operation. For level CC, multigrade oils are available but they are not widely used due to higher costs and general reluctance to the use of polymer. Today, there are no multigraded level CD lubricants, and the technology for formulating them in a completely satisfactory way is not known. A new polymer providing completely acceptable multigraded 1-H and 1-G performance without introducing a severe cost penalty would represent a major breakthrough.
We have now discovered novel and new VI* improvers which provide a significant improvement in high temperature diesel engine operation. This advance has been achieved by incorporating antioxidant functionalities directly into a suitable polymer. As such, the antioxidant moiety prevents extensive oxidative decomposition of the polymer and also other fluid additives, thus reducing the tendency to lacquer, TGF formation and land deposits. Incorporation of the subject polymers of this invention in diesel lubricants greatly enhances the possibility of the adoption of multigraded oils for both mild and severe diesel operation. FNT *VI is the abbreviation for viscosity index in this application.
One of the objects of the present invention is the use of antioxidant-containing VI improvers in crankcase lubricant formulations to increase the stability and improve the performance of such lubricants under high temperature engine operating conditions.
Another object is a method of preparing an antioxidant-containing VI improver polymer by esterifying a carboxylic acid-containing polymer with an N-methyl hydroxyethyl amide-containing antioxidant.